With his partner, David Gold, Sullivan's first business venture was selling soft pornography photos. They expanded into sex shops, adult magazines and several low-budget blue movies, making Sullivan a millionaire by the age of 25.[5] By the late 1970s, he was in control of half of the UK adult magazine market, including major titles such as Playbirds and Whitehouse,[6] 80% of the adult mail order market, and 150 shops.

In 1982, Sullivan was convicted of living off immoral earnings of prostitutes. He successful appealed and was released after serving 71 days in prison.[7] Sullivan explained that he did not feel embarrassed about the initial source of his early fortunes. "I've made a lot of people happy,” he said. “If I was an arms manufacturer or a cigarette manufacturer, and my products killed millions of my clients, I'd have a bit of doubt about the whole thing. I was a freedom fighter. I believe in the right of adults to make their own decisions."

In 2007, Sullivan expressed his first desire to sell his share in Birmingham City and openly admitted three reasons for a possible departure.[9] “One, the geographical distance. I've said for years the journey to Birmingham is killing me. Two, I think deep down the public have had enough of us. They think we should have mortgaged our houses to buy more players to compete with Chelsea and Arsenal. The honeymoon is long over and we're at the divorce stage now, unfortunately. And I also feel we've had no support from Birmingham Council."

After 16 years at the club, Birmingham managing director Karren Brady and Chairman David Sullivan agreed to step down from their posts on completion of Carson Yeung's takeover in Autumn 2009.[10]

Then manager Alex McLeish was quick to praise the Sullivan-Gold duo in the press for providing the club with financial stability, admitting he had a huge amount of respect for them and what they had achieved.[11]

On Sullivan’s departure from Birmingham he expressed his intentions to remain in football. Less than one year on in January 2010 he and David Gold acquired a 50% share in West Ham United giving them operational and commercial control and valuing the Premier League club at £105 million. Karren Brady, who had also played a significant role in turning around City’s fortunes, joined the Hammers as vice-chairman.

On the day the takeover was confirmed, an emotional Sullivan said: “It's going to take time to sink in. We've been wanting to sit here for 20 years and together we owned 27 per cent of the club 22 years ago and it's taken us 22 years to get to where want to be.
Both me and David are supporters, I went to university here and I lived in Hornchurch. David lived 50 yards from the ground for 20 years of his life and played for West Ham's youth team. We just want to be here where we've always wanted to be. There is no other club we would want to be at so for us we have come home and that's what it's all about."[12]

Within five months the Sullivan and Gold duo pledged a further commitment to the club and on 25 May 2010 they increased their stake to 60% (Sullivan with 30%). Vice-chairman Karren Brady later explained the move gave the football club "the much-needed stability."[13]

In September 2012 Sullivan revealed that together with Gold they were personally funding the club’s transfer business[14] because the ‘cupboard was left bare’ by its former Icelandic owners. He told the Daily Mail that ‘as a business venture, buying West Ham made no sense at all.’ He said, ‘Not the deal we did. The club was £120 million in debt with very few assets. It was a mess. But David [Gold] and I are not in it for the money. We want to put the best possible team on the pitch for the supporters of West Ham because we are supporters too.’

As part of his long term aspirations for the club Sullivan backed West Ham’s bid to move into the Olympic Stadium in Stratford.[15] On 22 March 2013, West Ham secured a 99-year lease deal, with the stadium planned to be used as their home ground from the 2016–2017 season.[16] In July 2013 Sullivan became the largest single shareholder of West Ham United acquiring a further 25% of shares in the club.[17] In March 2018, there were protests against Sullivan at the London Stadium during a 3-0 home defeat to Burnley. There were four pitch invasions and Sullivan was escorted from his seat before the end of the match.[18] Sullivan was also hit by a coin thrown by one of the supporters.[19]

On 26 March 2018 an HMRC tax tribunal ruled that Sullivan had used West Ham United to avoid paying £700,000 tax for his own family business, Conegate Ltd. Sullivan used Conegate to buy £2m of shares in the holding company that owns West Ham. The same day the shares were converted to "deferred shares", deemed practically worthless and sold back for £1 back to the holding company. Conegate thereby the £2m loss to reduce its tax bill.[20]

Sullivan lives in Birch Hall, near Theydon Bois, Essex. It was built by Ashby and Horner to Sullivan's design in 1992 and set in around 12 acres (49,000 m2) of land, at a cost of £7.5m for the land and building. This is in addition to a commercial property empire worth an estimated £500m. His girlfriend is Emma Benton-Hughes, the sister of Jonny Trunk. The couple have two children, David and Jack.[5] Jack Sullivan frequently provides updates on West Ham's transfer dealing via Twitter with his tweets being used by the press for information on possible transfers.[21] In 2013, he apologised after tweeting "I am very sorry about this news Chamakh has sign a 6 month loan deal! not my pick" relating to the loan signing of Marouane Chamakh from Arsenal.[22] In June 2015, Jack Sullivan was given his own news column on West Ham's website, www.whufc.com.[23] In May 2017, Jack Sullivan was appointed as managing director of West Ham United Women.[24]

Sullivan is a philanthropist and has been a patron of Prostate Cancer UK for over 15 years. He is also an active supporter of the Teenage Cancer Trust along with several other charities nationwide. Sullivan celebrated West Ham’s promotion to the Premier League in May 2012 by donating a five figure sum to a charity for children with autism.[25]

^Stephen Brook, press correspondent It is reported that he bought back the titles from the Administrators for £50,000 in May 2011 and the paper resumed publication. (8 August 2007). "Sullivan sells stake in Sport titles". London: Guardian. Retrieved 2010-04-28.CS1 maint: Multiple names: authors list (link)